UV-LED and display

ABSTRACT

A UV-LED is disclosed. The UV-LED includes a sapphire substrate, a u-GaN buffer layer formed on the sapphire substrate, an n-GaN contact layer formed on the u-GaN buffer layer, an InGaN light emitting layer formed on the n-GaN contact layer, and a p-GaN layer formed on the InGaN light emitting layer. The UV-LED has a quadrate planar shape with at least one side having a chip size of 50 μm or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application Nos.2018-181017 filed on Sep. 26, 2018, and 2018-199891 filed on Oct. 24,2018. These applications are hereby incorporated by reference in theirentirety including the specification, claims, drawings, and abstract.

TECHNICAL FIELD

The present disclosure relates to a UV-LED and a display.

BACKGROUND

With an improvement in output and efficiency of UV-LEDs which emit lightin the ultraviolet region, UV-LEDs are used as substitutes for UV lamps,and application of UV-LEDs is expanding to a wide variety of fields.Examples of the substitutes include high-resolution light sources suchas microscopes and exposure machines; chemical excitation light sourcesused for light-curing of resin and medical biology; spectral excitationlight sources used for bank bill recognition, DNA chips, andenvironmental measurement; and light sources for hygienic purposes suchas sterilization and disinfection. Besides these examples, UV-LEDs areused as phosphor excitation light sources such as illumination anddisplays.

Particularly, in the application as displays, micro LED displays areattracting attention as next-generation displays following organic ELs,which have disadvantages in lifetime and temperature rise. Currently,the following three techniques have been developed predominately: the 3LED method to which three types of LED chips; that is, red, blue, andgreen are applied; the blue LED method in which red and green phosphorsare excited by a blue LED; and the UV-LED method in which red, green,and blue phosphors are excited by a UV-LED.

WO 2011/027511 discloses a liquid crystal display device which includesan ultraviolet LED; a green phosphor selected from trivalent cerium- orterbium-activated rare earth boride phosphors; a blue phosphor selectedfrom divalent europium-activated halophosphate phosphors or divalenteuropium-activated aluminate phosphors; and a red phosphor selected fromeuropium-activated lanthanum oxysulfide phosphors or europium-activatedyttrium oxysulfide phosphors.

Among the aforementioned three techniques, the UV-LED method has anadvantage in that the method hardly affects emission colors, offers highcolor reproducibility due to high excitation efficiency, and facilitatescolor balance and synchronization, which leads to easy control. However,the UV-LED method is still not satisfactory in luminous efficiency.

SUMMARY

An object of the present disclosure is to provide a technique forUV-LEDs that enables further improvement in luminous efficiency andreduces a rate of power output decrease when current density increases.

A UV-LED according to the present disclosure includes a buffer layer, acontact layer formed on the buffer layer, a light emitting layer formedon the contact layer, and a patterned sapphire substrate on which thebuffer layer is formed. The UV-LED has a quadrate planar shape with atleast one side having a chip size of 50 μm or less. In an embodiment ofthe present disclosure, the light emitting layer may be an InGaN lightemitting layer.

A display according to the present disclosure includes the UV-LED; a redphosphor excited by light from the UV-LED; a green phosphor excited bylight from the UV-LED; and a blue phosphor excited by light from theUV-LED.

According to the present disclosure, it is possible to further improveluminous efficiency and to reduce a rate of power output decrease whencurrent density increases.

BRIEF DESCRIPTION OF DRAWINGS

Embodiment(s) of the present disclosure will be described by referenceto the following figures, wherein:

FIG. 1A is a view illustrating the principle of a display according toan embodiment;

FIG. 1B illustrates emission spectra of phosphors according to theembodiment;

FIG. 2 illustrates an arrangement of a 385 nm UV-LED chip according tothe embodiment;

FIG. 3 illustrates an arrangement of a 400 nm UV-LED chip according tothe embodiment;

FIG. 4 illustrates square flip chips according to the embodiment;

FIG. 5 illustrates rectangular flip chips according to the embodiment;

FIG. 6 illustrates an emission spectrum of a 385 nm chip according tothe embodiment;

FIG. 7 illustrates an emission spectrum of a 400 nm chip according tothe embodiment;

FIG. 8 illustrates luminous intensities of the 385 nm chips and the 400nm chips according to the embodiment;

FIG. 9 illustrates I-L characteristics of the 385 nm chips according tothe embodiment;

FIG. 10 illustrates I-L characteristics of the 400 nm chips according tothe embodiment;

FIG. 11 illustrates I-V characteristics of the 385 nm chips according tothe embodiment;

FIG. 12 illustrates I-V characteristics of the 400 nm chips according tothe embodiment;

FIG. 13 illustrates VF characteristics of the 385 nm chips and the 400nm chips according to the embodiment;

FIG. 14 illustrates a spectrum for each chip size of the 385 nm chipaccording to the embodiment;

FIG. 15 illustrates a spectrum for each chip size of the 400 nm chipaccording to the embodiment;

FIG. 16 illustrates yield for each chip size according to theembodiment;

FIG. 17A and FIG. 17B each illustrate an arrangement of a PSS accordingto the embodiment;

FIG. 18 illustrates a luminous intensity of each substrate according tothe embodiment; and

FIG. 19 illustrates a luminous intensity of each substrate according tothe embodiment and a luminous intensity of each sample without an SLSlayer.

DESCRIPTION OF EMBODIMENTS

Embodiment(s) of the present disclosure will now be described withreference to the drawings.

FIG. 1A illustrates the principle a UV-LED display according to thisembodiment. A plurality of UV-LED chips 12 are formed on a base 10, anda red phosphor 14, a green phosphor 16, and a blue phosphor 18 arestacked on the UV-LED chips 12. The red phosphor 14 is, for example,LOS:Eu; the green phosphor 16 is, for example, BAM:Eu, Mn; and the bluephosphor 18 is, for example, BAM:Eu, but the present disclosure is notlimited thereto. Herein, LOS represents La₂O₂S, and BAM represents (Ba,Mg) Al₁₀O₁₇.

FIG. 1B illustrates emission spectra of the red phosphor 14, the greenphosphor 16, and the blue phosphor 18. In FIG. 1B, (a) illustrates anemission spectrum of the blue phosphor 18, (b) illustrates an emissionspectrum of the green phosphor 16, and (c) illustrates an emissionspectrum of the red phosphor. Note that each broken line represents aspectrum of excitation light and that each solid line represents theemission spectrum.

The UV-LED chips 12 of practical use have a chip size of about 250 μm to2 mm. The UV-LED chips 12 are mounted on a CAN package, a surface mountdevice (SMD) package, or directly on a substrate. A rated current isabout 20 mA to 1 A, and luminous efficiency is over 50% at 365 nm but isstill not satisfactory.

Accordingly, the inventors have focused on the wavelength and chip sizeof the UV-LED chips 12. In this embodiment, the wavelength and chip sizeof a UV-LED are optimized to achieve further improvement in luminousefficiency of the UV-LED and to reduce a rate of power output decreasewhen current density increases.

FIG. 2 and FIG. 3 each illustrate an arrangement of a UV-LED chip inthis embodiment. FIG. 2 illustrates a UV-LED chip 1 having an emissionwavelength of 385 nm, and FIG. 3 illustrates a UV-LED chip 2 having anemission wavelength of 400 nm.

In FIG. 2, the UV-LED chip having an emission wavelength of 385 nm isobtained by stacking a u-GaN buffer layer 22 and a (GaN;Si) n-contactlayer 24 on a sapphire substrate 20 and by stacking an InGaN lightemitting layer thereon. Specifically,

an (AlInGaN)/(InGaN;Si) n-superlattice structure (SLS) layer 26,

an (InGaN/AlGaN) multiple quantum well (MQW) light emitting layer 28,

a p-AlGaN;Mg layer 30,

an (AlGaN;Mg/GaN;Mg) p-SLS layer 32, and

a p-GaN (GaN;Mg) layer 34 are stacked in order,

using an MOCVD apparatus. Herein, for example, (GaN;Si) representsSi-doped GaN. Furthermore, an ITO 36 is vapor-deposited by remote plasmadeposition (RPD), an n-pad layer 40 and an under barrier metal (UBM) 42are vapor-deposited by electron beam (EB), and an SiO₂ layer 38 isvapor-deposited by plasma-enhanced chemical vapor deposition (PECVD).More specifically, the u-GaN buffer layer 22 includes a low-temperatureu-GaN layer and a high-temperature u-GaN layer, and the u-GaN bufferlayer 22 is also referred to as an underlayer or a buffer coat.

Meanwhile, in FIG. 3, the UV-LED chip having an emission wavelength of400 nm is obtained by stacking a u-GaN buffer layer 22 and a (GaN;Si)n-contact layer 24 on a sapphire substrate 20, and on the resultinglaminate,

an (InGaN/AlGaN) MQW light emitting layer 28,

a p-AlGaN;Mg layer 30, and

a p-GaN (GaN;Mg) layer 34 are stacked in order,

using the MOCVD apparatus. An ITO 36 is vapor-deposited by remote plasmadeposition (RPD), an n-pad layer 40 and an under barrier metal (UBM) 42are vapor-deposited by electron beam (EB), and an SiO₂ layer 38 isvapor-deposited by plasma-enhanced chemical vapor deposition (PECVD).

The 385 nm and the 400 nm chips are similar in basic epitaxialstructure, but with regard to the (InGaN/AlGaN) MQW light emitting layer28 in the 385 nm chip, from a relationship of the band gap energy, theAl content is higher and the In content is smaller than those of the(InGaN/AlGaN) MQW light emitting layer 28 in the 400 nm chip. Inaddition, an In composition of the (InGaN/AlGaN) MQW light emittinglayer 28 is about 8% in the chip having an emission wavelength of 385 nmand about 15% in the chip having an emission wavelength of 400 nm. Toincrease n and p carriers, the chip having an emission wavelength of 385nm is provided with the superlattice structure (SLS) layers 26 and 32.The following values are a composition ratio and a film thickness ofeach layer in the UV-LED chips having an emission wavelength of 385 nmor 400 nm.

<385 nm>

u-GaN layer: 3.2 μm

(GaN;Si) n-contact layer: 2.7 μm

(Al_(0.20)In_(0.01)Ga_(0.79)N)/(In_(0.01)Ga_(0.99)N;Si) n-superlatticestructure (SLS) layer: 1.5 nm/1.5 nm×50 pairs

(In_(0.08)Ga_(0.92)N/Al_(0.20)Ga_(0.80)N) multiple quantum well (MQW)light emitting layer 1.8 nm/15 nm×3 pairs

p-Al_(0.25)Ga_(0.75)N layer: 20 nm

(Al_(0.20)Ga_(0.80)N;Mg)/(GaN;Mg layer) p-superlattice structure (SLS)layer: 0.8 nm/0.8 nm×30 pairs

p-GaN layer: 25 nm

ITO layer: 100 nm

SiO₂ layer: 500 nm

<400 nm>

u-GaN layer: 3.2 μm

(GaN;Si) n-contact layer: 2.7 μm

(In_(0.15)Ga_(0.85)N/Al_(0.10)Ga_(0.90)N) multiple quantum well (MQW)light emitting layer: 2.0 nm/15 nm 3 pairs

p-Al_(0.20)Ga_(0.80)N layer: 20 nm

p-GaN layer 25 nm

ITO layer: 100 nm

SiO₂ layer: 500 nm

In this embodiment, flip chips of eight totally different sizes arefabricated to optimize the chip size in such an epitaxial structure inconsideration of influences of the chip size. Herein, each flip chip isfabricated by undergoing a step of separation in which the MQW lightemitting layer 28 and the p-GaN layer 34 are formed and then thesapphire substrate 20 is etched. Then, each flip chip undergoes MESA,formation of the n-pad layer 40, formation of the SiO₂ passivation layer38, and formation of the under barrier metal (UBM) 42.

FIGS. 4 and 5 are photographs of flip chips in plan view. FIG. 4illustrates flip chips having a square planar shape, and FIG. 5illustrates flip chips having a rectangular planar shape. With regard tothe flip chips having a square planar shape,

<Square>

24 μm×24 μm

48 μm×48 μm

72 μm×72 μm

144 μm×144 μm

288 μm×288 μm

flip chips of these five different sizes are fabricated. With regard tothe flip chips having a rectangular planar shape,

<Rectangle>

12 μm×48 μm

24 μm×48 μm

24 μm×72 μm

flip chips of these three different sizes are fabricated.

With regard to the flip chips of eight different sizes fabricated foreach of the 385 nm and the 400 nm chips, emission spectra andintensities are measured with a prober, and also, shifts in voltagerelative to an injection current (I-V characteristics) and shifts inluminous intensity relative to an injection current (I-Lcharacteristics) are measured.

FIG. 6 illustrates an emission spectrum when an IF (forward current):278 μA is applied to a 24 μm×24 μm chip or the minimum area of the 385nm chip. The emission wavelength is 384.72 nm, and the half-value widthis 10.25 nm, which indicates a normal spectral waveform.

FIG. 7 illustrates an emission spectrum when an IF (forward current):278 μA is applied to a 24 μm×24 μm chip or the minimum area of the 400nm chip. The emission wavelength is 400.7 nm, and the half-value widthis 12.26 nm, which also indicates a normal spectral waveform.

FIG. 8 illustrates results of luminous intensities of the 385 nm chipsand the 400 nm chips measured at rated current density of 25.5 (A/cm²).In the figure, the chip size is taken along the horizontal axis, and theluminous intensity (a.u.) is taken along the vertical axis.

In all the chip sizes, the luminous intensity and the luminousefficiency are higher in the 385 nm chips than in the 400 nm chips. Inaddition, in both chips having a wavelength of 385 nm or 400 nm, theluminous intensity tends to increase with a reduction in the chip size.Particularly, with regard to the 385 nm chips, the luminous intensitymarkedly improves with a chip size of 24 μm×72 μm or less, and withregard to the 400 nm chip, the luminous intensity markedly improves witha chip size of 48 μm×48 μm or less. As described above, the chip size inthe related art is about 250 μm to 2 mm Therefore, the aforementionedchip sizes are prominently smaller than the chip size in the relatedart, which allows us to call them microchip sizes. The reason why theluminous intensity and the luminous efficiency prominently improve inmicro-sized chips is that a chip with a smaller size has a shorterdistance of a diffusion current, and causes an increase in emissionrecombination and an improvement in internal quantum efficiency.Furthermore, a chip with a smaller size has a shorter distance by whichlight emitted from a light emitting layer due to emission recombinationis taken to the outside, and produces an improvement in extractionefficiency.

FIGS. 9 and 10 illustrate I-L characteristics when a current is appliedto the chips of eight different sizes fabricated for each of the 385 nmand the 400 nm chips. FIG. 9 illustrates the I-L characteristics of the385 nm chips when current density is increased from 25.5 (A/cm²) to357.1 (A/cm²). FIG. 10 illustrates the I-L characteristics of the 400 nmchips when current density is increased from 25.5 (A/cm²) to 357.1(A/cm²).

Although a droop phenomenon is found in these I-L characteristics, allthe chips show good characteristic results. In the droop phenomenon, theluminous efficiency decreases at high current density. In addition, whencomparing the 385 nm chips and the 400 nm chips, the I-L characteristicsare lower in the 400 nm chip than in the 385 nm chip. This is becausethe In composition in the light emitting layer (InGaN) is higher in the400 nm chips than the 385 nm chips, and the 400 nm chips are inferior tothe 385 nm chips in crystallinity of the light emitting layer. When thecurrent density is low, the 400 nm chips offer high luminous efficiencydue to unevenness of the In composition, but when carrier concentrationincreases with an increase in current density, the 400 nm chips areeasily influenced by crystal defects inside the light emitting layer andby an increase in regions without emission recombination. Accordingly, achip having a lower In composition has a lower rate of power outputdecrease relative to current density.

FIGS. 9 and 10 show that the chips having a square planar shape aresuperior in I-L characteristics to the chips having a rectangular shape.In addition, the figures show that the I-L characteristics are morelinearly increased in the 385 nm chips than in the 400 nm chips.

FIGS. 11 and 12 illustrate I-V characteristics when a current is appliedto the chips of eight different sizes fabricated for each of the 385 nmand the 400 nm chips. FIG. 11 illustrates the I-V characteristics of the385 nm chips, and FIG. 12 illustrates the I-V characteristics of the 400nm chips. Leaks in low current regions are not found in either type ofchip, and each chip exhibits normal I-V characteristics. In both typesof chips, relatively small chips; that is, 24 μm×24 μm and 12 μm×48 μm,are compared with a relatively large chip; that is, 288 μm×288 μm. Thiscomparison shows that voltage of the relatively smaller chips rises at arelatively small current. In addition, the comparison shows that thechips having a square planar shape are superior in I-V characteristicsto the chips having a rectangular shape.

FIG. 13 illustrates VF (forward voltage) at 25.5 (A/cm²) in chips ofeight different sizes fabricated for each of the 385 nm and the 400 nmchips. The VF of each chip is 3.4 to 3.5 V.

In this manner, in a UV-LED chip having an emission wavelength of 385 nmand one having an emission wavelength of 400 nm, making the chip sizesmall; specifically, forming the chip to have a quadrate planar shapewith at least one side having a chip size of 50 μm or less,conspicuously improves the luminous intensity and luminous efficiency.More specifically, in a UV-LED chip having an emission wavelength of 385nm, at least one side is preferably 30 μm or less, and in a UV-LED chiphaving an emission wavelength of 400 nm, at least one side is preferably50 μm or less.

Furthermore, in UV-LEDs, the lower the In composition, the lower therate of power output decrease with respect to current density.Accordingly, a chip having an emission wavelength of 385 nm has a lowerrate of power output decrease with respect to current density than achip having an emission wavelength of 400 nm. This fact indicates that aUV-LED with a lower In composition and a shorter emission wavelengththan a 385 nm chip; for example, a UV-LED having an emission wavelengthof 365 nm, has a much lower rate of power output decrease with respectto current density, and that the greater the reduction in chip size, thegreater the improvement in luminous intensity and luminous efficiency.In short, a UV-LED with a lower In composition and a shorter emissionwavelength has advantages as a display.

FIG. 14 illustrates a spectrum for each chip size of the UV-LEDs havingan emission wavelength of 385 nm. FIG. 15 illustrates a spectrum foreach chip size of the UV-LEDs having an emission wavelength of 400 nm.Even when changing the chip sizes, no difference is found in thewaveform in any of the UV-LEDs having an emission wavelength of 385 nmor 400 nm, and no difference is found in the luminous intensity of thedeep level of each GaN layer around 500 nm to 500 nm. This result showsthat an emission spectrum depends on crystallinity of an epitaxiallygrown layer and hardly depends on chip sizes and that even when reducinga chip size; specifically, even when forming a chip to have a quadrateplanar shape with at least one side having a chip size of 50 μm or less,the chip emits light normally without problem.

FIG. 16 illustrates yield for each chip size of the UV-LEDs having anemission wavelength of 385 nm or 400 nm. Five chips of different sizes,24 μm×24 μm, 48 μm×48 μm, 72 μm×72 μm, 144 μm×144 μm, and 288 μm×288 μm,are prepared as the chips having a square shape. Among these chips, thenumber of products having good electrical characteristics and appearanceare counted. FIG. 16 illustrates a rate of the good products to allchips. As shown in the figure, the yield improves with a reduction inthe chip size. A possible reason for this result is that dust on asurface and an extraordinary epitaxial growth are about 50 μm at amaximum, but most of them are 50 μm or less and crystal defects are muchsmaller than 50 μm, which reduces the number of smaller-sized chipsdetermined to be defective relative to the total number of chips. From aviewpoint of the yield, it is desirable to employ a UV-LED having aquadrate planar shape with at least one side having a chip size of 50 μmor less.

In the above embodiment, the flat sapphire substrate 20 is used as theUV-LED having an emission wavelength of 385 nm or 400 nm, but apatterned sapphire substrate (PSS) may also be employed instead of theflat sapphire substrate 20.

FIG. 17A and FIG. 17B each illustrate an arrangement of a PSS. FIG. 17Ais a top view, and FIG. 17B is a side view. In each view, conicalpatterns are formed on a surface of a sapphire substrate. Each patternhas a height a=2.0 μm, a diameter b=3.75 μm, a pitch c=4.0 μm, and aspace d=0.25 μm, but is not necessarily limited thereto. Patternsapplied to a sapphire substrate reduce defect density of GaN crystalsgrown thereon and improve luminous efficiency of a light emitting layer.Furthermore, designing an optimum pattern shape enables efficientreflection of light emitted from a light emitting layer into an elementto the outside of the element, which reduces internal losses of thelight (rate at which the light changes to heat). A PSS is treated byforming a photoresist mask on a flat sapphire substrate and performingICP dry etching.

FIG. 18 illustrates luminous intensity of each chip size when the flatsapphire substrate and the PSS are used for UV-LEDs having an emissionwavelength of 385 nm or 400 nm. The figure illustrates the luminousintensity when rated current density 25.5 A/cm² is applied to theUV-LEDs, and “Flat” represents the flat sapphire substrate, while “PSS”represents the patterned sapphire substrate. Focusing on the UV-LEDshaving an emission wavelength of 385 nm, a chip with a smaller size hashigher luminous intensity, and the luminous intensity increases in “PSS”than “Flat” in all chip sizes. Similarly, in the UV-LED having anemission wavelength of 400 nm, a chip with a smaller size has higherluminous intensity, and the luminous intensity increases in “PSS” than“Flat” in all chip sizes. Accordingly, using a PSS and setting the chipsize to 50 μm or less causes more conspicuous improvement in luminousefficiency.

In this embodiment, UV-LEDs having an emission wavelength of 385 nm or400 nm are exemplified. However, the emission wavelength may be changedby changing a composition ratio of a light emitting layer, and thechanged emission wavelength is applicable to a UV-LED having an emissionwavelength from about 385 nm to 400 nm. Although the minimum chip sizeof each UV-LED in this embodiment is 24 μm×24 μm, each UV-LED may have achip size of about 10 μm×10 μm depending on manufacturing conditions,and such a UV-LED likewise offers improved luminous efficiency.

While micro LED displays for use as displays are attracting attention asnext-generation displays, their luminous efficiency is not sufficient.In particular, no sufficient study has been conducted aboutcharacteristics of micro LEDs including an InGaN light emitting layer; arelationship between chip sizes and luminous efficiency is now beingenergetically studied.

It should be particularly noted that a relationship between the chipsize of a micro LED including an InGaN light emitting layer and luminousefficiency is complicated; to achieve higher resolution displays, simplereduction in a chip size would not suffice. For example, a paperentitled “Electro-optical size-dependence investigation in GaN micro-LEDdevice”, Anis Daami et. al, 790/SID 2018 DIGEST, describes as follows:

The maximum external quantum efficiency is lowered in a non-negligiblemanner when the size decreases.

The optical current-density threshold shifts towards high current levelswhen the size decreases.

Drastic effect of μLED size reduction on external quantum efficiency isrecognized when the size approaches sub-micron dimensions.

Droop need to be as low as possible to enhance μLED optical emission athigh current levels.

When the current density is 10 A/cm², for example, the luminousintensity decreases as the chip size decreases from 500 μm, 50 μm, to 5μm,

μLED size effects on luminance and efficiency are importantspecifications to address, and issues to comprehend and optimize.

As described in the paper, a simple reduction in the chip size resultsin a decrease in the external quantum efficiency and a decrease in theluminous intensity. It has therefore been understood that a simplereduction in a chip size to achieve higher resolution lowers theluminous intensity and shifts the optical current-density towards highcurrent levels, which makes it difficult to obtain sufficient luminance.The inventors have found that not only a simple reduction in a chip sizebut also introduction of an SLS layer may regulate a decrease in theluminous intensity and may further enable an increase in the luminousintensity with a decrease in a chip size. While it is known that SLSlayers, which are known themselves, increase the external quantumefficiency of LEDs, effects of SLS layers on the chip size have not beenrecognized. The present embodiment first reveals effects of an SLS layeron a chip size, particularly a chip size of 50 μm or less.

FIG. 19 illustrates a relationship between luminous intensity and chipsizes of chips including chips without an SLS layer.

In FIG. 19, “385 nm (Flat/no-SLS)” is a sample having an emissionwavelength of 385, and including a flat sapphire substrate and an InGaNlight emitting layer disposed between a single layer of n-type cladlayer (Al_(0.1)Ga_(0.9)N;Si 195 nm) which is not an SLS and a singlelayer of p-type clad layer (Al_(0.1)Ga_(0.9)N;Mg 22 nm) which is not anSLS. The remaining samples are the same as those shown in FIG. 18. Thecurrent density is 25.5 A/cm².

The samples including an n-type clad layer and a p-type clad layer buthaving no SLS structure tend to show luminous intensity that isdecreased or substantially constant as the chip size decreases. Inparticular, in comparing samples having chip sizes of 72 μm×72 μm and 48μm×48 μm having no SLS layer with those samples having an SLS layer, inthe former samples without an SLS layer, the luminous intensity lowerswith a decrease in size. In contrast, chips having a chip size of 50 μmor less with an SLS layer have an increased luminous intensity and showexcellent characteristic which could not have been anticipated by thoseskilled in the art.

Causes of drastic differences in the chip sizes of 50 μm or less betweenpresence and absence of an SLS layer are not necessarily clear. Onepossible explanation is as follows: in the absence of an SLS layer, acarrier density is reduced and a chip with a smaller size has a shorterdistance of a diffusion current and therefore is more easily subjectedto effects of a reduction in the carrier density, causing a decrease inemission recombination to lower luminous intensity, whereas presence ofan SLS layer may effectively regulate these disadvantages.

In the absence of an SLS layer, the luminous intensity is lowered as thechip size decreases; therefore, micro LED devices having sufficientluminous efficiency cannot be obtained. Meanwhile, forming an n-type SLSlayer and a p-type SLS layer with an InGaN light emitting layerinterposed therebetween and setting the chip size to 50 μm or lessincreases the luminous efficiency. Thus, the UV-LED chip of the presentembodiment accomplishes notable advantages which could not have beenanticipated by those skilled in the art.

The invention claimed is:
 1. A UV-LED comprising: a buffer layer; acontact layer formed on the buffer layer; an n-type SLS layer formed onthe contact layer; an InGaN light emitting layer formed on the n-typeSLS layer; a p-type Mg-doped AlGaN layer formed on the InGaN lightemitting layer; and a p-type SLS layer formed on the p-type Mg-dopedAlGaN layer, wherein the UV-LED has a quadrate planar shape with atleast one side having a chip size of 50 μm or less, and wherein theUV-LED has an emission wavelength of 385 nm to 400 nm.
 2. A displaycomprising: the UV-LED according to claim 1; a red phosphor excited bylight from the UV-LED; a green phosphor excited by light from theUV-LED; and a blue phosphor excited by light from the UV-LED.
 3. TheUV-LED according to claim 1, further comprising: a patterned sapphiresubstrate on which the buffer layer is formed.
 4. The UV-LED accordingto claim 1, wherein the chip size of the quadrate planar shape is anyone of 24 μm×24 μm, 12 μm×48 μm, 24 μm×48 μm, 24 μm×72 μm, and 48 μm×48μm.
 5. The UV-LED according to claim 1, wherein the buffer layerincludes a low-temperature u-GaN layer and a high-temperature u-GaNlayer.
 6. A display comprising: the UV-LED according to claim 5; a redphosphor excited by light from the UV-LED; a green phosphor excited bylight from the UV-LED; and a blue phosphor excited by light from theUV-LED.